Now You See It, Now You Don't

The signal enhancement properties of a single-loop delayline integrator (DLI) are derived for operation in the presence of correlated noise. The signal is assumed periodic with a period equal to the delay of the DLI and present only for a finite time interval. The noise is assumed additive, zero mean, statistically independent from the signal, and present for an infinite time. The noise also is assumed covariance stationary with a specified auto correlation function. Enhancement is defined as the ratio of the increase in average signal power to the increase in average noise power due to integration. The results are general in that the enhancement is derived in terms of the autocorrelation coefficients of the input noise. Plots of enhancement versus number of signal observations and feedback gain are shown for uncorrelated and exponentially correlated noise. The plots show graphically the optimum choices of feedback gain and number of signal observations for a specified auto correlation function and signal observation time. When strongly correlated noise, characteristic of 10-GHz backscatter from salt flats at low incidence angles, rather than uncorrelated noise is assumed present at the input of the DLI, an example, using DLI parameters typical of modern airborne radars, shows a degradation in the maximum achievable enhancement of 9 dB.     

On the Acquisition Time for an Apollo Ranging Code

An Apollo ranging system is considered whose phase reference is obtained by a phase-locked loop for bit synchronization. The bit phase reference is noisy, and the error probability for the ranging code is shown to depend on the input signal energy per bit to noise density ratio. The procedure of computing the acquisition time for the ranging code is then presented and the acquisition time for a lunar ranging code is plotted versus the input signal-to-noise density ratio.     

Power factor correction circuits with robust current control technique

A dynamically robust current control method to synthesize a sinusoidal input current for AC-to-DC converters with boost type topology is presented. Under this control strategy, the inductor current and the diode current of the boost converter are fed back and combined in a special way which makes the input current of the AC-to-DC converter stable and robust. The input current is solely determined by the reference current. When the reference current signal is derived from the sinusoidal input voltage, the input current is sinusoidal and in phase with the input voltage. Theoretical analysis is first provided. Small signal analysis shows that the current loop is inherently stable and has a fast dynamic response. Large signal analysis reveals that the control system is not affected by large disturbances in supply voltage or output load. Computer simulations have been carried out and experimental prototype models have been built to verify the analysis and demonstrate the feasibility of the control strategy. A power factor of 0.998 and a total harmonic distortion (THD) of 3.18% are measured.     

On Signal Fluctuations in Tracking Systems

All tracking systems suffer, to some extent, from signal level fluctuations. The signal fluctuations are modeled by a Gaussian process and the noise performance of both linear and nonlinear first-order tracking systems is considered. In the nonlinear case, the Ito versus Stratonovich calculus difficulty arises, but it is resolved to make sense physically. Several analytical results are obtained which are useful in analyses of tracking systems.     

The Effect of Hard Limiting an Angle-Modulated Signal Plus Noise

The effect of hard limiting an angle-modulated signal plus narrow-band Gaussian noise is analyzed. Several examples are considered?sinusoidal angle modulation, Gaussian angle modulation, and biphase angle modulation. The general conclusion is that when a zonal band-pass filter is used, which rejects dc and second harmonics, an angle-modulated signal plus Gaussian noise provides the same output signal-to-noise ratio as shown by Davenport for a CW signal plus Gaussian noise. However, when a narrow bandpass filter is used, which has a bandwidth approximately equal to the input angle-modulated signal, an angle-modulated signal plus Gaussian noise has a better output signal-to-noise ratio than a CW signal plus Gaussian noise.     

Some Matched Filter Configurations for Infrared Systems

An analysis of the output of three alternative matched filter configurations in an infrared scanning system model is presented. The sensor is corrupted by thermal noise, generation-recombination noise, photon noise, and modulation noise, the latter providing an extreme discoloration in the signal passband. Expressions for the signal voltage density spectrum, signal pulse shape, noise power spectrum, and average noise power at the matched filter output are derived where the integral evaluations attendant to these derivations do not appear elsewhere in the literature. The paper also provides graphical displays of the signal-to-noise power ratio at the filter output versus various system parameters, noise power spectrum out of the matched filter versus ?, and the signal pulse shape out of the filter versus time. Also included are discussions of practically realizable approximations to the matched filters and curve fitting techniques for the signal pulse shape function.     

航空发动机振动环境谱统计归纳方法及振动试验台复现

为满足航空发动机及机载产品研制过程贴近使用环境的振动考核试验需求,需根据发动机实测振动数据给出振动考核试验所需的输入谱图。依据GJB/Z 126-99中给出的环境测量数据归纳方法,建立了发动机实测振动环境谱统计归纳方法并通过程序实现。利用发动机多架次实测试飞振动数据统计归纳得到发动机测点位置的振动实测谱。基于能量等效及信号频域特征分布一致原则,将归纳得到的实测谱转化为可用于振动台输入的振动环境谱,并在振动台上进行了振动信号的复现试验。结果表明：振动台输出信号与发动机实测振动信号频域分布特征一致,在统计频率带宽范围内振动总量最大相差5.7%,证明了转化方法是合理的,为航空发动机机载设备贴近使用环境的振动考核试验方法提供了真实的输入谱图。     

The Calculation of Frequency-Modulation Distortion in Linear Networks

A numerical method is developed for the calculation of steady-state FM distortion in a linear passive network from the character of the FM signal input and the steady-state transfer characteristics of the network. The method of calculating FM distortion requires the solution of a finite set of linear equations which is accomplished readily by a digital computer. To illustrate the use of the method, the FM distortion introduced by a Chebyshev-response bandpass filter is calculated for a range of network and input signal parameters.     

A UHF True Logarithmic IF Amplifier

A UHF ?true logarithmic IF? which yields a carrier output proportional in amplitude to the logarithm of the carrier input has been constructed. Although this ?true? form has been known for a number of years, it appears to have been neglected in development. Good performance has been obtained over a 60-dB input signal range with rise and fall times of 2 ns per 20 dB. A high-level, detected dc coupled output without the use of any dc amplifier is obtained. Coherent signal processing of the output carrier is possible, and other important advantages to this type of logarithmic IF are described.     

Adaptive Arrays for Multiple Simultaneous Desired Signals

Steered beam adaptive arrays for multiple simultaneous desired signals are discussed. It is shown that the performance of a steered beam adaptive array depends upon the range of input signal strengths and the choice of the steering vector. Optimum steering vectors for various input signal strengths are given. All choices of steering vectors are equally effective in the rejection of jammers.     

On the Mean Frequency Measurement System Using Correlation Detection

This paper is concerned with the problem of measuring the mean frequency of the power spectrum of a zero-mean, stationary, narrowband Gaussian random signal in the presence of additive Gaussian noise. Signal-to-noise ratios at the output of the mean frequency measurement system using correlation detection are analyzed in terms of input signal-to-noise ratio, input signal and noise bandwidths, and integration time. The results obtained are verified experimentally, and a comparison with a conventional zero-crossing detector is also made.     

A Clutter Reduction Technique for Random Signal Radars

The performance of certain radars is degraded in environments with significant clutter returns, and since the clutter is signal-generated, increasing the transmitted power does not improve the situation. However, changing the pulse width and pulse period of the transmitted signal can increase the input signal-to-interference ratio. In this correspondence, the transmitted signal is made up of pulses of random waveforms and the receiver is a correlator where the reference signal extends over many pulses. An expression for input signal-to-interference ratio as a function of pulse width and period is obtained for the case of a distributed target. This expression could be maximized by any of several methods, but to further elucidate the clutter reduction technique, contour plots of the input signal-to-interference ratio are presented.     

Phase-Locked Loop Behavior near Threshold

The phase-locked loop behavior is analyzed following the quasilinearization Booton's method. When the loop is locked on an unmodulated input signal with a static phase error, the phase detector nonlinearity produces an interaction between the static phase error and the voltage-controlled-oscillator (VCO) noise phase fluctuations. Formulas allowing one to compute the static phase error increase and the VCO phase variance increase are derived. When the input signal is phase modulated, there is an interaction between the static phase error, the VCO noise phase fluctuations, and the input signal phase modulation. Formulas are obtained that allow one to compute the loop loss of performances (static phase error increase and VCO phase variance increase) and the coherent phase demodulator output signal-to-noise ratio decrease. Finally, a slight modification to Booton's procedure is proposed, leading to results in better agreement with experimental data.     

Detectors for Multiomial Input

The binary detection problem is considered. Under an arbitrary noise environment, the input sample space can be transformed into a multinomial vector. Based on observations of this vector, the Neyman-Pearson optimal detector is developed for a known signal. When the signal strength is unknown, the likelihood ratio principle is followed to obtain consistent tests which use the Pearson's chisquare statistic. The resulting detectors are compared to others in terms of asymptotic relative efficiency under some actual noise distributions.     

Hard-Limiter Output Autocorrelation Function: Gaussian & Sinusoidal Input

An expression is derived for the autocorrelation function of the output of a hard limiter whose input is stationary Gaussian noise with zero mean plus independent random-phase sinusoidal signal. The output spectrum may then be evaluated. This spectrum is extremely useful in understanding the properties of a filter-limit-filter-detect signal processor whose signal input is an actual sinusoid, or when a sinusoid is used as a test signal.     

A digital quadrature demodulation system

Existing digital quadrature demodulation techniques sample the input at either 2B Hz or 4B Hz, select the even samples as the in-phase (I), and interpolate the odd samples to give the quadrature (Q), output. The signal bandwidth is B. We propose a demodulation system to produce I and Q samples at arbitrary sampling rate greater than 2B Hz. The system eliminates the IF downconversion step with a special sampling scheme. The even samples correspond to the I component, while the Q components are the filtered output. The filter can be a lowpass or least squares (LS) filter. The lowpass filter design is based on trade-offs between the filter length and the degree of oversampling. It produces similar results as previous work when the sampling rate is 2B Hz or 4B Hz. Unlike existing methods which assume the input is white, a LS filter, on the other hand, can make use of input signal characteristics to achieve a better result. The higher the correlation in the input the larger the improvement. The cost for LS filtering is a coefficient update step if the input is time varying. A scheme to cancel dc offset from analog to digital (A/D) converters is also given     

Energy Detection of a Random Process in Colored Gaussian Noise

A likelihood receiver for a Gaussian random signal process in colored Gaussian noise is realized with a quadratic form of a finite-duration sample of the input process. Such a receiver may be called a "filtered energy detector." The output statistic is compared with a threshold and if the threshold is exceeded, a signal is said to be present. False alarm and detection probabilities may be estimated if tabulated distributions can be fitted to the actual distributions of the test statistic which are unknown. Gamma distributions were fitted to the conditional probability densities of the output statistic by equating means and variances, formulas for which are derived assuming a large observation interval. A numerical example is given for the case in which the noise and signal processes have spectral densities of the same shape or are flat. The optimum filter turns out to be a band-limited noise whitener. The factors governing false alarm and detection probabilities are the filter bandwidth, the sample duration, and the signal level compared to the noise. Two sets of receiver operating characteristic curves are presented to complete the example.     

Complexity analysis of wavelet signal decomposition andreconstruction

We give certain sequential and parallel algorithms and their computational analysis for signal decomposition and reconstruction based on wavelets. The signal decomposition (respectively, reconstruction) process is separated into two stages: the first is the preprocessing stage where certain constants are computed for implementation to prepare for the second stage in which signal decomposition (respectively, reconstruction) is performed. In the decomposition (respectively, reconstruction) stage, the input signal is transformed via different methods to compute the output signal without changing the setup initialized in the preprocessing stage. We describe certain sequential algorithms for both the preprocessing and the decomposition (respectively, reconstruction) stages, and parallel algorithms for the latter. The algorithms are finally illustrated for compactly supported spline-wavelets and are analyzed in detail in terms of the required arithmetic operations     

运载火箭电动伺服机构前馈自抗扰控制方法的设计

运载火箭伺服机构是火箭的执行机构,在工作过程中不仅要求伺服机构具有较好的阶跃响应和力矩抗扰性能,还要求伺服机构能够较好地跟踪箭载计算机发送的位置随动指令。常规的自抗扰控制(ADRC)建模时,将输入的微分量近似为0,使得输入时变信号时会产生建模误差,该误差无法通过扩张状态观测器(ESO)进行观测并补偿,导致系统的跟踪误差较大。针对常规自抗扰控制对时变信号跟踪误差较大的问题,提出了一种将位置输入微分前馈(PIDF)引入自抗扰控制的前馈自抗扰控制方法。通过理论推导和建模仿真得知,该方法可降低系统对正弦输入信号的跟踪误差并提高系统的动态特性,同时仍具有较强的抗干扰性能。最后通过试验验证了该方法的有效性。     

应用于航空变频电源系统的新型串沥混合有源滤波器

 目前变频交流(AC)电源系统开始逐渐应用到新型多电飞机(MEA)中,传统有源滤波器无法满足宽输入频率变化要求,为此需要设计一种适合于宽变频航空电源系统的有源滤波器。本文研究了一种应用于航空变频电源系统的新型串联混合有源滤波器(SHAPF),该有源滤波器直接根据基波电流进行补偿,针对各种频率输入均能起到较好的抑制谐波作用。讨论了这种新型SHAPF的变频谐波抑制原理,给出了该滤波器的控制系统详细设计过程,包括基于双数字信号处理器（DSP）的控制平台设计和逐点动态检测基波电流的算法设计,最后进行了样机实验,对比了加入滤波器前后系统输入电流实验波形和谐波分析,加入滤波器后系统输入电流总谐波失真度（THD）由174.00%变化至4.80%,滤波效果良好;在360 Hz,400 Hz和720 Hz不同输入频率进行了滤波实验,验证系统在宽变频输入条件下均具有良好控制性能,满足航空变频输入时输入功率因数校正要求。